The present invention relates to a flame-retardant epoxy resin composition superior in flame retardancy and safety, as well as to a varnish solution, a prepreg and a laminate all made with the composition.
In an epoxy resin composition required to have flame retardancy for fire prevention, there have generally been used a halogen-based flame retardant and, as an auxiliary flame retardant, antimony trioxide.
Use of such a flame retardant and such an auxiliary flame retardant in an epoxy resin composition gives rise to a safety problem and, moreover, incurs metal corrosion. For these reasons, such an epoxy resin composition has had problems in application. For example, when such an epoxy resin composition is used as an insulating material for electronic part, there have occurred, in some cases, reduction in corrosion resistance of wiring particularly at high temperatures and resultant deterioration of reliability of the electronic part. Therefore, it has been desired to develop an epoxy resin composition using neither halogen-based flame retardant nor antimony trioxide.
As a means for imparting flame retardancy to an epoxy resin composition without using any halogen-based flame retardant or the like, it is known to compound a metal hydroxide in an epoxy resin composition. Expression of flame retardancy by metal hydroxide is due to the ignition-preventing action by the temperature decrease (endothermic reaction) of cured resin and is regarded as an auxiliary means for imparting flame retardancy. Therefore, use of a large amount of the metal hydroxide is required in order to obtain sufficient flame retardancy by such an endothermic reaction. This results in a significant reduction in moldability, etc. when such an epoxy resin composition is used in applications such as electronic part and the like, and the actual use of the epoxy resin composition has been difficult.
When a metal hydroxide is used in a large amount in an epoxy resin composition, particularly for production of a flame-retardant laminate by impregnation of the resulting epoxy resin composition into a glass fiber or the like and subsequent curing of the composition, various problems arise. These problems are described below.
The first problem is reduction in the processability of the laminate obtained. With respect to this, for example, the pages 270 to 271 of xe2x80x9cLatest Techniques of Flame Retardant and Flame Retardation (published on Jul. 30, 1999 from Technical Information Institute Co., Ltd.)xe2x80x9d describe that addition of a large amount (75% by mass relative to total epoxy resin composition) of aluminum hydroxide can achieve UL 94 V-0, but such an addition amount is unrealistic in practical application and xe2x80x9cgives rise to problems in punching and drilling in printed wiring board production as well as in soldering in parts mountingxe2x80x9d.
The second problem is increase in dielectric constant and reduction in moisture resistance and soldering heat resistance. While these properties must be maintained at sufficient levels in laminate applications, metal hydroxides are hygroscopic and have high dielectric constants; therefore, addition of a large amount of a metal hydroxide invites reduction in the above properties.
Thus, with the means of adding a metal hydroxide to an existing epoxy resin, it has been difficult to achieve a high degree of flame retardancy while keeping the properties required for laminate applications, at high levels.
Meanwhile, various studies have been made on imparting flame retardancy by change of the molecular structure of an epoxy resin or a curing agent. In JP-A-11-140277 is disclosed a flame-retardant-free epoxy resin composition used for encapsulation of semiconductor device, containing, as essential components, a phenolic resin of novolac structure containing, in the molecule, a biphenyl derivative and/or a naphthalene derivative, an epoxy resin of novolac structure containing, in the molecule, a biphenyl derivative and/or a naphthalene derivative, an inorganic filler and a curing accelerator.
In the above epoxy resin composition used for encapsulation of semiconductor device, the phenolic resin and the epoxy resin both having, in the structure, a polycyclic compound(s) such as biphenyl derivative, naphthalene derivative and/or the like are reacted to form a crosslinked structure; therefore, the composition, when ignited, shows rubber-like expansion at the surface to form a foamed layer. This foamed layer shields the supply of heat and oxygen to the unburnt portion, whereby a high degree of flame retardancy is expressed.
The above resin composition, however, is designed so as to fit an application of semiconductor device encapsulation and therefore, when used for applications such as laminate and the like, shows no sufficient flame retardancy. This is owing to the fact that since a laminate contains therein a substrate such as a woven and nonwoven glass fabric which prevents the deformation (expansion) of the resin component, formation of a sufficient amount of a stable foamed layer in the laminate during its ignition is difficult.
The present invention has been made in view of the above-mentioned situation and aims at providing a flame-retardant epoxy resin composition showing a high degree of flame retardancy and safety both of which have been unobtainable with the prior art.
The present invention aims, in particular, at allowing a flame-retardant epoxy resin composition used for production of a laminate, to have a high degree of flame retardancy while having the properties to be possessed by the laminate, i.e. processability, dielectric properties, moisture resistance, soldering heat resistance and the like.
In order to achieve the above aim, the present invention provides a flame-retardant epoxy resin composition comprising an epoxy resin, a curing agent and a metal hydroxide,
wherein the curing agent is a phenolic resin (C) containing, in a molecular chain, a structural unit derived from a phenol (A) and a structural unit derived from an aromatic compound (B) other than the phenol (A).
The present invention also provides a flame-retardant epoxy resin composition comprising an epoxy resin, a curing agent and a metal hydroxide,
wherein the epoxy resin is a novolac epoxy resin (D) obtained by glycidyletherifying a phenolic hydroxyl group of a phenolic resin (C) containing, in a molecular chain, a structural unit derived from a phenol (A) and a structural unit derived from an aromatic compound (B) other than the phenol (A).
The present invention also provides a flame-retardant epoxy resin composition comprising an epoxy resin, a curing agent and a metal hydroxide,
wherein the curing agent is a phenolic resin (C) containing, in a molecular chain, a structural unit derived from a phenol (A) and a structural unit derived from an aromatic compound (B) other than the phenol (A), and
the epoxy resin is a novolac epoxy resin (D) obtained by glycidyletherifying a phenolic hydroxyl group of a phenolic resin (Cxe2x80x2) containing, in a molecular chain, a structural unit derived from a phenol (Axe2x80x2) and a structural unit derived from an aromatic compound (Bxe2x80x2) other than the phenol (Axe2x80x2).
In the flame-retardant epoxy resin composition, the phenol (A), the aromatic compound (B) and the phenolic resin (C) may be the same with or different from the phenol (Axe2x80x2), the aromatic compound (Bxe2x80x2) and the phenolic resin (Cxe2x80x2), respectively. Incidentally, the descriptions on the phenol (A), the aromatic compound (B) and the phenolic resin (C), made in this specification apply also to the phenol (Axe2x80x2), the aromatic compound (Bxe2x80x2) and the phenolic resin (Cxe2x80x2).
The present invention also provides an epoxy resin varnish solution obtained by dispersing the above flame-retardant epoxy resin composition in an organic solvent; a prepreg obtained by impregnating the above flame-retardant epoxy resin composition into a substrate and curing the impregnated composition; and a laminate obtained by laminating a plurality of the prepregs and hot-pressing them.
In the present invention, a phenolic resin and an epoxy resin having the above-mentioned particular structure, and a metal hydroxide are used in combination, whereby a high degree of flame retardancy is realized. By using, in particular, a combination of a phenolic resin of the above particular structure and an epoxy resin of the above particular structure, even higher flame retardancy is obtained.
The flame-retardant epoxy resin composition of the present invention comprises a phenolic resin (C) containing, in the molecular chain, structural units derived from a phenol (A) and structural units derived from an aromatic compound (B); and/or an epoxy resin (D) obtained by subjecting the phenolic hydroxyl groups of the phenolic resin (C) to etherification with glycidyl; and further comprises a metal hydroxide. Owing to the synergistic effect of these components, a high degree of flame retardancy is obtained as described below.
As mentioned in the section of the prior art, the cured material of an epoxy resin composition in which a phenolic resin (C) and/or an epoxy resin (D) both containing an aromatic compound (B) in the molecular skeleton, form a crosslinked structure, generates a decomposition gas inside when ignited, gives rise to rubber-like expansion of the surface resin layer, and form a stable foamed layer; thereby, flame retardancy is expressed. With such an action alone, however, no sufficient flame retardancy is obtained in the case of an epoxy resin composition used in, for example, a laminate containing a substrate such as a woven and nonwoven glass fabric, which hinders the deformation (expansion) of the resin component, because, in such an epoxy resin composition, efficient formation of a foamed layer capable of showing a high degree of flame retardancy is difficult.
Hence, in the flame-retardant epoxy resin composition of the present invention, there are used an epoxy resin of the above-mentioned particular structure, a curing agent of the above-mentioned particular structure and a metal hydroxide. Owing to the synergistic effect of these components, strikingly high flame retardancy unobtainable with the prior art is obtained. The reasons are not clear; however, the present flame-retardant epoxy resin composition is considered to express its flame retardancy based on the following mechanism.
The cured material of the present flame-retardant epoxy resin composition, when ignited, gives rise to thermal decomposition of the metal hydroxide to generate water vapor. This water vapor allows the cured resin material softened by the heat of ignition to deform and expand, whereby formation of a foamed layer is promoted. Consequently, a sufficient amount of a foamed layer can be formed during ignition even when the present epoxy resin composition is used in a structure containing a substrate such as a woven and nonwoven glass fabric, which hinders the deformation (expansion) of the resin component. Further, this foamed layer has a high hot strength and is hardly broken by heat because of the particular crosslinked structure brought about by a particular epoxy resin and a particular curing agent; moreover, is filled with water vapor, etc. inside; therefore, can effectively shield heat and oxygen and can effectively act as a ignition-preventing layer.
The metal hydroxide, when ignited, is converted into a metal oxide and this metal oxide remains uniformly in the resin material. The metal oxide is considered to act as a support and allow the foamed layer to have a uniform size. Thus, the metal hydroxide is considered to allow the foamed layer to have a preferred structure and accordingly higher ignition preventability.
As mentioned above, in the present invention, an epoxy resin of particular structure, a curing agent of particular structure and a metal hydroxide are used in combination; thereby, formation of a flamed layer of high hot strength is promoted, the foamed layer is filled with water vapor inside and is allowed to have ignition preventability, and the metal oxide converted from the metal hydroxide becomes a support and allows the foamed layer to have a structure suitable for ignition prevention; as a result, a high degree of flame retardancy is considered to be realized.
In the present invention, since a metal hydroxide is used for such a purpose, its addition amount can be small to obtain sufficient flame retardancy, as compared with the prior art in which a metal hydroxide is used simply for temperature lowering.
In the present epoxy resin composition, addition of, in particular, a silicone compound having a branched structure main chain and containing, in the structure, aromatic compound-derived group can allow the epoxy resin composition to have significantly improved flame retardancy. The added silicone compound reacts with the epoxy resin and the curing agent in the epoxy resin composition to form a flame-retardant substance superior in thermal decomposition resistance; therefore, when the cured resin material is ignited, a foamed layer more resistant to foam breakage is formed and higher flame retardancy can be attained. Further, the addition of the silicone compound enables reduction in the amount of the metal hydroxide used, which allows effective prevention of the reduction in the moldability and electrical properties (dielectric properties) of the epoxy resin composition, because of lowering the metal hydroxide.